1. Field of the Invention
The present invention relates to a one roll stack calender for paper web deformation More particularly, the present invention relates to a calender which applies a compressive stress to a paper web in its working nips for a period of time (i.e., a dwell time).
2. Discussion of the Related Art
Calenders for the treatment of paper web are well known. See for example, "Die neuen Superkalanderkonzepte" [The New Supercalender Concepts], Sulzer Papertec Company, May 1994. Such calenders are used to finish coated and uncoated paper webs, e.g., printing papers or silicon base papers. Typically, calenders include metal rollers ("hard rollers") having a smooth, hard surface, to provide a smooth and gloss-like finish to the paper web. Calenders also include rollers that are fabricated with an elastic or soft surface, which evenly compress the paper web. Such soft rollers are commonly referred to as "soft rollers". A changeover nip is provided in calenders to effect even treatment of both sides of the paper web.
Calenders typically include 12 to 16 rollers, wherein the changeover nip is located in the lower half of the calender roller stack. Such calenders are configured to operate so that the paper web, which is traversing from the top toward the bottom of the stack, is deformed to a lesser extent at the top than it is at the bottom. This is because the loading on the stack increases toward the bottom due to the cumulative weight of the rollers and any parts connected to them, such as overhanging weights. This results in a compressive stress and/or dwell time in the working nips that increases from the top of the stack toward the bottom of the stack. Thus, such prior art calenders are disadvantageous in that they have a very tall construction height and are very expensive due to the large number of rollers.
Therefore, it is an object of the present invention to overcome the shortcomings of the prior art calenders and provide an improved calender that has a shorter construction height and lower manufacturing and operating costs.
It is another object of the present invention to provide a calender that provides customary finishing results to a paper web while maintaining a maximum allowable compressive stress in the lowest working nip due to a reduced number of rollers and therefore reduced cumulative weight on the rollers in comparison to prior art calenders.